1. Field of the Invention
The present invention relates to an optical intensity modulator and a method for fabricating the same, and more particularly, to an integrated optical intensity modulator having an optical waveguide in which an area of discontinuous refractivity having a staggered pattern is induced by applying a voltage, and to a method for fabricating the same.
2. Description of the Related Art
Integrated optics is a technology for fabricating various optical devices based on an optical waveguide, on a substrate. This can simplify arrangement of optical devices and manufacture of devices having many functional devices in a small area, reducing production costs. Also, electrodes are formed around the optical waveguide and thus an electric field is produced only in an optical waveguide area, to control the passage of light waves with a low driving voltage. A typical material of the integrated optical substrate is a ferroelectric such as LiNbO.sub.3 or LiTaO.sub.3, or an electro-optic polymer.
The optical intensity modulator is a device for switching on and off a light wave transmitted along the optical waveguide, using a voltage, and is used as a main element of an optical communication system and an optical sensing system. The optical intensity modulator can be one of two types, one of which uses phase modulation, e.g., a Mach-zehnder interferometric modulator or a directional coupler switch, and the other of which modulates the refractive index by abruptly changing it, e.g., a cutoff modulator.
The structure of the cutoff modulator is simple enough to be fabricated cheaply, and thus it can be tuned to be suitable for various applications. The cutoff modulator can be used, for example, for realizing a linear optical modulator which has no direct current (DC) bias and a DC-drift effect which may occur in setting an operation point of an optical modulator with a DC voltage.
FIG. 1A is a perspective view of a conventional optical intensity modulator. The optical intensity modulator of FIG. 1A includes a LiNbO.sub.3 substrate in which crystal is cut in the Z direction (Z-cut) or a Z-cut LiTaO.sub.3 substrate 100, a channel optical waveguide 102 manufactured on the substrate 100 by an annealed proton exchange method, and electrodes 104 capable of applying an electric field to the optical waveguide 102 and the substrate 100. At this time, a buffer layer 106 such as SiO.sub.2 is formed on the optical waveguide to suppress ohmic loss of a light wave passing through the optical waveguide caused by the electrode, and then an electrode formed of Cr and Au is formed. If a voltage V.sub.a is applied to the electrode to apply an electric field to the optical waveguide in the +Z direction, a change .DELTA.n of the refractive index is as follows. ##EQU1## where n denotes the refractive index, r.sub.33 denotes an electro-optical coefficient and E.sub.z denotes a Z-direction component of an applied electric field. According to Formula 1, if the electric field is applied in the +Z direction, the refractive index is reduced, and if in the -Z direction, it is increased. Thus, if the electric field in the +Z direction is applied to the optical modulator of FIG. 1A, the refractive index of the optical waveguide is reduced as shown in FIG. 1C, and if the light wave enters the modulation area as shown in FIG. 1B, scattering loss occurs due to mode-shape mismatching. Reference numeral 108 of FIGS. 1A and 1B denotes a modulation area. Reference numerals of FIGS. 1B and 1C common to FIG. 1A represent the same elements. Here, the amount of scattering loss is determined by a difference between a guided mode distribution f.sub.1 of an optical waveguide input portion and a guided mode distribution f.sub.2 of a modulation area. If guided mode power of the optical waveguide input portion is P.sub.1, guided mode power of the modulation area is P.sub.2, and a cross-sectional area of the optical waveguide is s, scattering loss due to mode-shape transition is as follows. ##EQU2## Here, * indicates a complex conjugate. The modulation depth of the optical intensity modulator can be obtained using Formula 2. In order to obtain the maximum modulation depth, I.sub.21 becomes zero in applying a voltage, and thus a mode distribution f.sub.2 of the modulation area must be scattered on the entire surface of the substrate. That is, the optical waveguide must be cut off. In order to make waveguide cutoff easy when an electric field is applied to the waveguide, it is common to set initial guiding conditions of the optical waveguide to near the cutoff condition. However, if the guiding conditions of the optical waveguide are set to near the cutoff condition, the insertion loss of the optical modulator is increased.
Guided mode distribution in the modulation area is similar to that of the input portion of the optical waveguide, requiring a high driving voltage. That is, the guided mode distribution of the modulation area, like that of the guided mode distribution of the input portion of the optical waveguide, is symmetrical around a center point of the optical waveguide, and peak points of the mode distributions coin.sub.-- cide, making it difficult to effectively reduce the I.sub.21 value of Formula 2. Thus, a high voltage must be applied to obtain a required extinction ratio of approximately 20 dB or higher.